This application is a 371 national filing claiming priority from international application PCT/EP2008/002772, filed Apr. 8, 2008, and the entire contents of that international application are incorporated herein by reference.
The object of the invention relates to membrane housing elements that are constructed from an inner element, a fiber wrapping, and a casing, and to a method for their manufacture.
The preparation of potable water, and water purification are becoming increasingly important. Usually, hyperfiltration also referred to as reverse osmosis, or nano- and ultrafiltration is used for the preparation of potable water by desalting sea water and by purifying brackish water. To carry out the osmosis or for the filtration of water and other liquids, membranes in the form of membrane elements are frequently used. Usually, spiral membranes are used here, where the membrane film is wound around the permeate tube. These wound membrane elements are inserted in a pressure resistant housing, which withstands the usual operating pressures, namely pressures of the order of magnitude of 16-100 bar. The usual membrane housings, as used in sea water or brackish water desalting installations, accommodate several membrane elements that are arranged in rows and interconnected. The membrane housings for reverse osmosis here present lengths of, for example, 1-7 m, and they have diameters that are suitable for receiving 2.5, 4, 8 and 16 inch membrane elements. In nano- and ultrafiltration in particular, the sizes vary as a function of the given area of application. A membrane element that can be used for reverse osmosis is known, for example, from EP 0 601 301 B1.
The requirements that are placed on the membrane housings in which the membrane elements are introduced are demanding due to the pressure load. In contrast to pressure pipes, for example, membrane housings must be able to withstand greater forces, because the operating pressure is 16-84 bar for the reverse osmosis process, for example. In addition, the cylinder of the membrane housing must absorb the forces of the closed head ends, which are superposed on the radial pressure particularly in the end areas. In contrast to the pressure pipes, the membrane housing therefore must also present high precision, so that the membrane element can be inserted into the membrane housing. For membrane housings, the manufacturing tolerances are thus substantially lower, and the housings must present a nearly circular cross section and the same diameter at every place along the axis. To achieve this high precision, membrane housings have been manufactured largely manually to date with calibrated steel mandrels or using calibrated inner liners.
A corresponding method is known, for example, from EP 0 422 457 B1. The method described there has the following course, for example:
a. The prefinished inner liner pipes are sawed to the desired length and mounted on a support mandrel.
b. The elements that have been sawed to size are then clamped on a machine, which rotates the inner liner uniformly. A polyamide-soaked glass fiber is applied manually to the pipe, while the latter rotates mechanically. The pipe then rotates until several completely covering layers have been built up, with the carriage moving back and forth. The glass fiber-containing mass is hardened on the inner liner. The hardening is carried out either by UV light or in an oven, over several hours under rotation. Once the membrane housings have hardened, the ends are trimmed and polished.
c. In an additional step, the polyamide composition is coated optionally with a UV hardening polymer composition, to produce a visually appealing casing.
Although this method leads to membrane housing elements that present a nearly identical diameter over the entire membrane housing length, and no ovalities, this method allows the manufacture of only a comparatively small number of pieces per day. Because the method presents several steps that have to be carried out manually, it is also comparatively cost-intensive. The numerous work steps entail much manual work, and they are also very time consuming because of the complicated handling of the heavy and bulky parts. Moreover, the individual work stations take up much space for the transport and the storage of the parts.
In addition, during the manufacture of the corresponding membrane housings, one must ensure that the internally located pipe which comes in contact with the desalted water does not contain any substances that are a health threat, and could be washed out of the pipe. In addition, the entire housing must not allow the passage of light and UV radiation, because the latter can potentially foster the growth of algae or bacteria.
The manufacturing methods available to date require, moreover, that the tubular membrane housings must be manufactured in different lengths and diameters as a function of the application purpose and site of application. Accordingly, many different types and sizes of membrane housings and tubular membrane housing elements that are designed for different pressure levels, lengths, and diameters must be kept in stock.
Simple pressure pipes are used, for example, for gas pipelines or water lines, and therefore buried in the ground. Accordingly, only minimal demands are placed on the optical surface constitution. In contrast, in the case of membrane housings, high demands are placed on optical quality. The surface must be smooth and as uniform as possible, and it must present a visually appealing appearance. Therefore, in the case of membrane housings, in contrast to pressure pipes, an optically high-quality casing is necessary.